Drug therapy for mental illnesses dates back to the late 1800s, when lithium was first tried for those institutionalized with severe conditions. But little was known about how the drug actually worked, and at the time, it was likely much more a mask of symptoms than a real treatment. That being said, all drug therapy seeks to limit symptoms and allow a person to be functional, so for the late 19th century, that was a decent attempt on the part of the medical community.

The use of opioid alkaloids began in the early 20th century, as psychoactive drugs were experimented with to treat the clinically depressed, with limited results. Real medical advances with anti-depressants did not take place until the 1950s when more was learned about serotonin and nor-epinephrine and how to manipulate them for mood effect. Since it was a new drug therapy, it was a bit clumsy, using too high of doses, or chemical combinations that blunted the symptoms but not optimally.

In the late 1980s the science got much better, and started to use different combinations of drugs, dosages and regimens that more specifically targeted different kinds of mental illnesses, wth fewer side effects.

The link below to psychiatrist.com has a good timeline of the development of these drugs and a lot of good information, but it is in .pdf format.